In a separate incident a hoard of similar devices was seized in the Netherlands after being smuggled in from the former Yugoslavia. Also found in the same arms cache was a gun so tiny it had been disguised as a key-ring.

And German police have intelligence reports of the mobile-guns being used by criminals in their country.

"We are obviously very concerned," says Wolfgang Bicke, a spokesman for the German Police Union (Gewerkschaft der Polizei). "We have good reason to suspect that a significant number of these weapons are entering Western Europe from the Balkans.

"We have alerted all our police forces and told them to be especially vigilant, and I know they have done the same in Holland and several other countries."

The mobile phone gun looks exactly the same as a normal mobile phone, with a keypad, liquid crystal display and aerial.

Inside, however, rather than wires, it contains a rudimentay spring-wound percussion mechanism which allows four .22 calibre bullets to be fired in quick succession from openings in the top the phone, each bullet being triggered by pushing a button on the keypad.

In the case of the Croation phones the weapon was actually fired by depressing the "connect" button, while buttons five to eight determined which of the four barrels was used.

The only noticeable difference to a real mobile phone is in terms of weight, the gun-phone being markedly heavier.

"This sort of technology has been around pretty much since guns were first invented," says British firearms expert Terry Gander. "During World War Two, for instance, secret service agents were using pistols disguised as fountain pens.

"Basically you can conceal a gun in anything that can house a short barrel. Lighters, briefcases, cameras, Walkmans - they've all been used before."

According to Gander, the majority of these type of weapons originate in the Balkans, especially Bulgaria.

"It's not confined to Bulgaria, but that does seem to be the main place. I've been to a lot of arms fairs there and seen these sort of things on display.

Wolfgang Bicke confirms Gander's views. "There are clearly established links between criminals living in the Balkans and their relatives in continental Europe," he explains. "There is constant traffic between the two areas.

"Not just in weapons, but in anything that will turn a profit. Drugs, women, pornography - you name it."

Although the gun-phones are more than capable of killing, their one drawback is that they are extremely inaccurate.

"It's really very simple technology," says Gander. "Almost as soon as the bullet leaves the muzzle it becomes unstable, making it accurate only up to a couple of metres at the most.

"Basically you'd have to go right up to your target and practically press the gun right against them for it to be effective."

Despite this, however, police forces across Europe are taking the threat of the mobile phone weapon very seriously indeed.

"It's a very worrying development," says Bicke. "How are officers expected to know whether the person they stop on the street is carrying a phone or something deadly?